Interference Cancellation/Mitigation Capable Receivers
In Universal Mobile Telecommunications System/High-Speed Downlink Packet Access (UMTS/HSDPA (several interference aware receivers have been specified for the User Equipment (UE). They are termed as ‘enhanced receivers’ as opposed to the baseline receiver (rake receiver). The UMTS enhanced receivers are referred to as enhanced receiver type 1 (with two-branch receiver diversity), enhanced receiver type 2 (with single-branch equalizer), enhanced receiver type 3 (with two branch receiver diversity and equalizer) and enhanced receiver type 3i (with two branch receiver diversity and inter-cell interference cancellation capability). The new receivers can be used to improve performance, e.g., in terms of throughput and/or coverage.
In Long Term Evolution Release-10 (LTE Rel-10), enhanced interference coordination techniques have been developed to mitigate potentially high interference, e.g., in a cell range expansion zone, while providing the UE with time-domain measurement restriction information. Further, for LTE Release-11 (LTE Rel-11), advanced receivers based on Minimum Mean Square Error-Interference Rejection Combining (MMSE-IRC) with several covariance estimation techniques and interference-cancellation-capable receivers are being currently studied. In future even more complex advanced receivers such as Minimum Mean Square Error-Successive Interference Cancellation (MMSE-SIC), which is capable of performing nonlinear subtractive-type interference cancellation, can be used to further enhance system performance.
Such techniques generally may benefit all deployments where relatively high interference of one or more signals is experienced when performing measurements on radio signals or channels transmitted by radio nodes or devices, but are particularly useful in heterogeneous deployments.
However, these techniques involve also additional complexity, e.g., may require more processing power and/or more memory. Due to these factors such receiver may be used by the UE for mitigating interference on specific signals or channels. For example a UE may apply an interference mitigation or cancellation technique only on data channel. In another example a more sophisticated UE may apply interference mitigation on data channel as well as on one or two common control signals; examples of common control signals are reference signal, synchronization signals etc.
It should be noted that the terms interference mitigation receiver, interference cancellation receiver, interference suppression receiver, interference rejection receiver, interference aware receiver, interference avoidance receiver etc are interchangeably used but they all belong to a category of an advanced receiver or an enhanced receiver. All these different types of advanced receiver improve performance by fully or partly eliminating the interference arising from at least one interfering source. The interfering source is generally the strongest interferer(s), which are signals from the neighbouring cells when the action is performed in the UE. Therefore a more generic term, ‘enhanced receiver’, which covers all variants of advanced receiver, is used hereinafter. Further, the corresponding interference handling techniques (e.g., interference cancellation, interference suppression, puncturing or interference rejection combining) for enhanced receivers are termed ‘enhanced receiver technique’ herein.
Heterogeneous Deployments
In 3rd Generation Partnership Project (3GPP), heterogeneous network deployments have been defined as deployments where low-power nodes of different transmit powers are placed throughout a macro-cell layout, implying also non-uniform traffic distribution. Such deployments are, for example, effective for capacity extension in certain areas, so-called traffic hotspots, i.e. small geographical areas with a higher user density and/or higher traffic intensity where installation of pico nodes can be considered to enhance performance. Heterogeneous deployments may also be viewed as a way of densifying networks to adopt for the traffic needs and the environment. However, heterogeneous deployments bring also challenges for which the network has to be prepared to ensure efficient network operation and superior user experience. Some challenges are related to the increased interference in the attempt to increase small cells associated with low-power nodes, aka cell range expansion; the other challenges are related to potentially high interference in uplink due to a mix of large and small cells.
According to 3GPP, heterogeneous deployments consist of deployments where low power nodes are placed throughout a macro-cell layout. The interference characteristics in a heterogeneous deployment can be significantly different than in a homogeneous deployment, in downlink or uplink or both.
Examples hereof with Closed Subscriber Group (CSG) cells are given in FIG. 1, where in case (a), a macro user with no access to the CSG cell will be interfered by the Home enhanced Node B (HeNB), in case (b) a macro user causes severe interference towards the HeNB and in case (c), a CSG user is interfered by another CSG HeNB. Heterogeneous deployments, however, are not limited to those with CSG involved.
Another example is illustrated in FIG. 2, where the need for enhanced Inter-Cell Interference Coordination (ICIC) techniques for DownLink (DL) is particularly crucial when the cell assignment rule diverges from the Reference Signal Received Power (RSRP)-based approach, e.g. towards pathloss- or pathgain-based approach, sometimes also referred to as the cell range expansion when adopted for cells with a transmit power lower than neighbor cells. In FIG. 2, the cell range expansion of a pico cell is implemented by means of a parameter Δ. The pico cell is expanded without increasing its power, just by changing the reselection threshold, e.g., UE selects cell of pico Base Station (BS) as the serving cell when RSRPpico+Δ≧RSRPmacro, where RSRPmacro is the received signal strength measured for the cell of macro BS and RSRPpico is the signal strength measured for the cell of pico BS.
Transmit Patterns and Measurement Patterns for Enhanced ICIC (eICIC)
To facilitate measurements in the extended cell range, i.e., where high interference is expected, the standard specifies Almost Blank Subframe (ABS) patterns for eNodeBs and restricted measurement patterns for UEs. A pattern that can be configured for eICIC is a bit string indicating restricted and unrestricted subframes characterized by a length and periodicity, which are different for Frequency Division Duplex (FDD) and Time Division Duplex (TDD) (40 subframes for FDD and 20, 60 or 70 subframes for TDD). Only DL patterns have been so far specified for interference coordination in 3GPP, although patterns for Uplink (UL) interference coordination are also known in prior art.
ABS pattern is a transmit pattern at a radio node transmitting radio signals; it is cell-specific and may be different from the restricted measurement patterns signaled to the UE. In a general case, ABS are low-power and/or low-transmission activity subframes. ABS patterns may be exchanged between eNodeBs via X2, but these patterns are not signalled to the UE, unlike the restricted measurement patterns.
Restricted measurement patterns (more precisely, “time domain resource restriction patterns” [TS 36.331]) are configured to indicate to the UE a subset of subframes for performing measurements, typically in lower interference conditions, where the interference may be reduced e.g. by means of configuring Multimedia Broadcast Single Frequency Network (MBSFN) subframes or ABS subframes at interfering eNodeBs.
Restricted measurement patterns may, however, be also configured for UEs with good interference conditions, i.e., receiving a measurement pattern may be not necessarily an indication of expected poor signal quality. For example, a measurement pattern may be configured for UE in the cell range expansion zone where typically high interference is expected, but a measurement pattern may also be configured for UEs located close to the serving base station where the signal quality is typically good which may be for the purpose of enabling a higher-rank transmission modes (e.g., rank-two transmissions).
Restricted measurement patterns are in general UE-specific, although it is known in prior art that such patterns may be broadcasted or multicasted. Three patterns are currently specified in the standard to enable restricted measurements:                Serving-cell pattern for Radio Link Monitoring (RLM) and Radio Resource Management (RRM) measurements,        Neighbor-cell pattern for RRM measurements,        Serving-cell pattern for Channel State Information (CSI) measurements.        
Transmit patterns and measurement patterns are means for coordinating inter-cell interference in wireless network and improve measurement performance. Alternatively or in addition to inter-cell interference coordination techniques, measurement performance may also be improved by using more advanced receiver techniques, e.g., interference suppression or interference cancellation techniques.
Large-Scale Channel and Propagation Properties
Some examples of large-scale channel/propagation properties and environment characteristics are:                Delay spread        Doppler spread        Doppler shift (aka frequency shift)        Average received power        Receive timing or propagation time        
Signals may arrive via different propagation paths. Receiving two signals with very different any of the above properties is typically more complex and resource demanding, e.g., in terms of memory, measurement time, sampling, processing time and resources (e.g., number of Fast Fourier Transforms (FFTs) in parallel), power, etc.
Transmit signal configuration (e.g., physical time and/or frequency resource configuration, mapping to transmit time and/or frequency resources, transmit power, etc.) is typically done to account for the properties above, e.g., extended cyclic prefix may be configured in cells with a large delay spread.
Cyclic Prefix
Cyclic prefix (CP) is a prefix of a symbol with a repetition of the end of the symbol. Although the receiver is typically configured to discard the CP samples, the CP often serves two purposes:                As a guard interval, it eliminates the inter-symbol interference from the previous symbol; and        As a repetition of the end of the symbol, it allows the linear convolution of a frequency-selective multipath channel to be modelled as circular convolution, which in turn may be transformed to the frequency domain using a discrete Fourier transform. This approach allows for simple frequency-domain processing, such as channel estimation and equalization.        
In order for the CP to be effective (i.e. to serve its aforementioned objectives), the length of the CP must be at least equal to the length of the multipath channel. Although the concept of CP has been traditionally associated with Orthogonal Frequency Division Multiplexing (OFDM) systems, the CP is now also used in single carrier systems to improve the robustness to multipath.
Extended CP length may be configured in cells with a large delay spread. Some examples of scenarios where the received signal could have components with longer delay, i.e., where the extended CP may be useful, are as follows:                large cells,        rural environment, mountain environments, etc.,        receiving signals from far-away cells (e.g., with positioning when many cells need to be detected),        network deployments with repeaters or relays,        indoor scenarios with many reflections and propagations paths, and        scenarios when MBSFN subframes are configured (with or without Multimedia Broadcast and Multicast Service (MBMS) traffic), etc.        
Cyclic Prefix in LTE
In LTE, normal CP or extended CP may be used, and the CP may be configured separately for DL and UL, i.e., CP in DL may be different from that of the CP in UL. The CP length determines the number of symbols in a slot, duration of transmitted signal/channels, and signal/channel mapping to time-frequency resource grid.
The DL transmission scheme is based on conventional OFDM using a cyclic prefix. The OFDM sub-carrier spacing is Δf=15 kHz. 12 consecutive sub-carriers during one slot correspond to one downlink Resource Block (RB). In the frequency domain, the number of resource blocks, NRB, can range from NRB-min=6 to NRB-max=110 per carrier or per cell in case of CA.
In addition there is also a reduced sub-carrier spacing Δflow=7.5 kHz, only for MBMS-dedicated cell.
In the case of 15 kHz sub-carrier spacing there are two cyclic-prefix lengths, corresponding to seven and six OFDM symbols per slot respectively.                Normal cyclic prefix: TCP=160×Ts (OFDM symbol #0), TCP=144×Ts (OFDM symbol #1 to #6),        Extended cyclic prefix: TCP-e=512×Ts (OFDM symbol #0 to OFDM symbol #5),        
where Ts=1/(2048×Δf).
In case of 7.5 kHz sub-carrier spacing, there is only a single cyclic prefix length TCP-low=1024×Ts, corresponding to 3 OFDM symbols per slot.
For UL transmission scheme, there are two CP lengths defined: normal CP and extended CP corresponding to seven and six Single-Carrier Frequency Division Multiple Access (SC-FDMA) symbol per slot, respectively.                Normal cyclic prefix: TCP=160×Ts (SC-FDMA symbol #0), TCP=144×Ts (SC-FDMA symbol #1 to #6),        Extended cyclic prefix: TCP-e=512×Ts (SC-FDMA symbol #0 to SC-FDMA symbol #5).        
Existing Signalling of the CP Information
For UL, the CP information may be sent to the UE (via Radio Resource Control (RRC) protocol) in a higher-layer parameter UL-CyclicPrefixLength comprised in Information Element (IE) RadioResourceConfigCommonSIB (e.g., in System Information Block 2 (SIB2)) and/or IE RadioResourceConfigCommon (e.g., in RRCConnectionReconfiguration message), which are used to specify common radio resource configurations in the system information and in the mobility control information, respectively, e.g., the random access parameters and the static physical layer parameters.
The information about whether normal or extended CP is used in DL and UL may also be exchanged between eNodeBs over X2 interface in Served Cell Information IE, but only as a part of TDD information and more specifically, as a part of the Special Subframe Info IE.
The CP length information may also be signalled to the UE in the Observed Time Difference Of Arrival (OTDOA) assistance information provided by the positioning node Evolved Serving Mobile Location Centre (E-SMLC) over LTE Positioning Protocol (LPP) for the OTDOA reference cell and also for all neighbour cells which have CP length different from that of the reference cell. Note also that the CP length of Cell-specific Reference Signal (CRS) may be different from that of the Positioning Reference Signals (PRS), e.g., the current standard says:                cpLength        This field specifies the cyclic prefix length of the neighbour cell PRS if PRS are present in this neighbor cell, otherwise this field specifies the cyclic prefix length of CRS in this neighbor cell.        
The PRS CP length information may also be signaled from eNodeB to E-SM LC provided in the IE OTDOA Cell Information over LPPa protocol.
The information about the DL CP length used in neighbour cells is not signalled to the UE. The UE therefore blindly detects the CP length of a neighbour cell during cell identification.
CP Length in MBSFN Subframes
Extended CP is always used in MBSFN subframes containing MBMS data. However, normal CP may be used, e.g., in blank MBSFN subframes. Further, as indicated above, CP length of CRS may be different from that of the PRS in the same MBSFN subframe.
MBSFN subframes with extended CP may be configured for various purposes. Further, except for MBMS data transmissions, they may be used for one or more purposes at the same time, e.g., as low-interference positioning subframes, low-interference subframes for backhaul signaling, and/or as ABS subframes with enhanced Inter-Cell Interference Coordination (eICIC). Further, there is no restriction to configure as low-interference subframes only blank MBSFN subframes or only normal subframes, e.g., low-interference subframes may be a mix of MBSFN and non-MBSFN subframes. In fact, it may be not even always possible to configure MBSFN subframes since they can only be configured in pre-defined MBSFN-configurable subframes.
CP Length in Positioning Subframes
Positioning subframes may be configured in subframes that use different CP length. Further, positioning signals need to be detected at multiple locations and the coverage of positioning signals does not need to be one-to-one mapped to cells for data transmissions, e.g., virtual cells may be created for positioning.
In addition to the explicit signalling of PRS CP length over LPPa and the CP length information over LPP, there exist also pre-defined rules related to the PRS CP length for OTDOA positioning:                PRS can only be transmitted in resource blocks in DL subframes configured for PRS transmission. If both normal and MBSFN subframes are configured as positioning subframes within a cell, the OFDM symbols in a MBSFN subframe configured for PRS transmission shall use the same CP as used for subframe #0. If only MBSFN subframes are configured as positioning subframes within a cell, the OFDM symbols configured for PRS in the MBSFN region of these subframes shall use extended CP length.        In a subframe configured for PRS transmission, the starting positions of the OFDM symbols configured for PRS transmission shall be identical to those in a subframe in which all OFDM symbols have the same CP length as the OFDM symbols configured for PRS transmission.        
Multi-Carrier or Carrier Aggregation Concept
To enhance peak-rates within a technology, multi-carrier or carrier aggregation solutions are known. For example, it is possible to use multiple 5 MegaHertz (MHz) carriers in High-Speed Packet Access (HSPA) to enhance the peak-rate within the HSPA network. Similarly in LTE for example multiple 20 MHz carriers or even smaller carriers (e.g. 5 MHz) can be aggregated in the UL and/or on DL. Each carrier in multi-carrier or carrier aggregation system is generally termed as a Component Carrier (CC) or sometimes is also referred to a cell. In simple words the component carrier (CC) means an individual carrier in a multi-carrier system. The term Carrier Aggregation (CA) is also called (e.g. interchangeably called) “multi-carrier system”, “multi-cell operation”, “multi-carrier operation”, “multi-carrier” transmission and/or reception. This means the CA is used for transmission of signaling and data in the uplink and downlink directions. One of the CCs is the Primary Component Carrier (PCC) or simply primary carrier or even anchor carrier. The remaining ones are called Secondary Component Carrier (SCC) or simply secondary carriers or even supplementary carriers. Generally the primary or anchor CC carries the essential UE specific signaling. The primary CC exists in both uplink and direction CA. The network may assign different primary carriers to different UEs operating in the same sector or cell.
Therefore the UE has more than one serving cell in downlink and/or in the uplink: one primary serving cell and one or more secondary serving cells operating on the PCC and SCC respectively. The serving cell is interchangeably called as Primary Cell (PCell) or Primary Serving Cell (PSC). Similarly the secondary serving cell is interchangeably called as Secondary Cell (SCell) or Secondary Serving Cell (SSC). Regardless of the terminology, the PCell and SCell(s) enable the UE to receive and/or transmit data. More specifically the PCell and SCell exist in DL and UL for the reception and transmission of data by the UE. The remaining non-serving cells on the PCC and SCC are called neighbor cells.
The CCs belonging to the CA may belong to the same frequency band (aka intra-band CA) or to different frequency band (inter-band CA) or any combination thereof (e.g. 2 CCs in band A and 1 CC in band B). The inter-band CA comprising of carriers distributed over two bands is also called as Dual-Band-Dual-carrier-High-Speed Downlink Packet Access (DB-DC-HSDPA) in HSPA or inter-band CA in LTE. Furthermore the CCs in intra-band CA may be adjacent or non-adjacent in frequency domain (aka intra-band non-adjacent CA). A hybrid CA comprising of intra-band adjacent, intra-band non-adjacent and inter-band is also possible. Using carrier aggregation between carriers of different technologies is also referred to as “multi-Radio Access Technology (RAT) carrier aggregation” or “multi-RAT-multi-carrier system” or simply “inter-RAT carrier aggregation”. For example, the carriers from Wideband Code Division Multiple Access (WCDMA) and LTE may be aggregated. Another example is the aggregation of LTE and Code Division Multiple Access 2000 (CDMA2000) carriers. For the sake of clarity the carrier aggregation within the same technology as described can be regarded as ‘intra-RAT’ or simply ‘single RAT’ carrier aggregation.
The CCs or the serving cells in CA may or may not be co-located in the same site or base station or radio network node (e.g. relay, mobile relay etc). For instance the CCs may originate (i.e. transmitted/received) at different locations (e.g. from non-located BS or from BS and Remote Radio Head (RRH) or Remote Radio Unit (RRU)). The well-known examples of combined CA and multi-point communication are Distributed Antenna System (DAS), RRH, RRU, Coordinated Multi Point (CoMP), multi-point transmission/reception etc. The invention also applies to the multi-point carrier aggregation systems.
The multi-carrier operation may also be used in conjunction with multi-antenna transmission. For example signals on each CC may be transmitted by the eNB to the UE over two or more antennas.